**Core Concept**
Gray (Gy) is a unit of ionizing radiation dose in the International System of Units (SI), representing the absorption of one joule of radiation energy per kilogram of matter. It's an essential concept in radiology and radiation therapy.

**Why the Correct Answer is Right**
One gray (Gy) is equivalent to the absorption of 100 rad by mass, which is a unit of absorbed radiation dose. This equivalence is crucial in understanding the effects of radiation on living tissues. The Gy is widely used in medical physics and radiation oncology to quantify the dose of radiation delivered to patients. In contrast, the rad is an older unit of absorbed radiation dose, and its use has been largely replaced by the Gy.

**Why Each Wrong Option is Incorrect**
**Option A:** 1 rad is incorrect because it represents a much smaller dose of radiation compared to 1 Gy. The Gy is a more precise and modern unit of radiation dose.
**Option B:** 10 rad is incorrect because it's still a much smaller dose of radiation compared to 1 Gy. The Gy is a more accurate representation of the absorbed radiation dose.
**Option D:** 1000 rad is incorrect because it represents a much larger dose of radiation compared to 1 Gy. This dose would be equivalent to 10 Gy, not 1 Gy.

**Clinical Pearl / High-Yield Fact**
When working with radiation doses, remember that 1 Gy is equivalent to 100 rad. This conversion factor is essential in radiation oncology and medical physics to ensure accurate dosing and minimize radiation exposure.

**✓ Correct Answer: C. 100 rad**